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Bouncy Tach in fourth

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M

mgrojor

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I Have a 70 MGB At about 3500 in fourth the tach seems to bounce foward about 500 RPM foward than the back . Through the gears or below 3500 this does not happen. the bouncing is not violent could there be a damper in the tach that could be causing this??
Thanks in Advance
Bob J
 
Bob, This is only food for thought. It could be the instrument voltage stabilizer sensing a much higher than normal voltage coming to it when you rev the engine over 3500 RPMs. As I said, this is only food for thought but, check to see if the voltage coming from the alternator is excessively high at or above 3500 RPM. The instrument voltage stabilizer works off of the bi metal strip principal by opening and closing a set of points in the unit. If the points are sticking and or bouncing it could cause a sensitive instrument, like a tach, to fluctuate. It could be that the instrument stabilizer is bad and or a bad alternator voltage regulator. /ubbthreads/images/graemlins/rolleyes.gif PJ
 
thanks for the help is there an particular place tha the stabilizer was put on can't seem to see it.
Thanks
Bob J
 
This is a sidebar question. I thought the MGB I worked on in high-school had a cable driven tach. What year did they change to an electric instrument?
 
Last cable tac was about 72 as I remember for spridgets anyway.
 
Timely post.
I brought our 71 to work tonight for the first time. While traveling on the highway, 62 mph, tach at 3500 it was fine. When I sped up at all, same as Bob.. Jumping around like crazy; slow down and nice and steady.
Maybe it is a good way to tell me when I am speeding. /ubbthreads/images/graemlins/smile.gif

Paul
 
Bob, The instrument stabilizer is usually located on the fire wall in side the car above your left foot when setting in the drivers seat. It's about 1 1/2 inches long and 1/2 inch wide or so and has three wires hooked to it. Held to the fire wall by one screw. I don't know if this is your problem, but it's a good start. Moss sells new ones.
70s MGBs have electronic tachs. The spedo is driven by cable. /ubbthreads/images/graemlins/cheers.gif PJ
 
[ QUOTE ]
This is a sidebar question. I thought the MGB I worked on in high-school had a cable driven tach. What year did they change to an electric instrument?

[/ QUOTE ]

I've never seen a cable driven tach MGB. I don't know for a fact they were always electronic but I suspect so. We'll need more feedback on this as I'm curious too.

The other possibility for a "dancing" tach is the points could be out of adjustment (if it HAS points /ubbthreads/images/graemlins/wink.gif).
 
I was distracted by my own question and didn't post a proper reply.

The voltage stabilizer supplies a controlled voltage for the fuel gauge and an electric temperature gauge. It does not supply the tach. The stabilizer shouldn't be affecting your tach. I'm sorry I can't contribute to what actually might be causing the bouncing.

DR, you made me go look. The MGB I worked on years ago was a '64. From the small number of sources I looked at, it appears that when BMC went to the 5 main bearing block they dropped the mechanical tach drive and were definitely using the RVI tachometers. Some sources I found indicated that the change to electric may have been as early as late 1964. If that's true it would have been before the introduction of the 5 main bearing block.
 
Thanks Doug! The '64 here (as far as I can tell) came with a 5-main and has the electronic unit fitted. Wiring ~looks~ original but(!)... I can't for the life of me ever recall seeing a B with a generator driven tach... and every 'A' I've ever seen had the mechanical unit fitted. The pre-'64 cars were likely "Hen's Teeth" though.

This stuff makes me crazy at times!
 
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